Control system for steplessly variable power transmission

Abstract

A control system for a steplessly variable transmission comprising a drive pulley and driven pulley formed with V-shaped grooves respectively, an endless belt engaged with the both grooves of the pulleys and tensioned between the pulleys to transmit engine torque therethrough, cylinders provided on the pulleys for being subjected to a hydraulic pressure to vary an effective diameter of the pulleys so that a speed ratio of the transmission is continuously changed, a hydraulic pressure supply device for introducing the hydraulic pressure into the cylinder means, a line pressure control device for controlling a line pressure of a hydraulic circuit which controls operation of the cylinders, a line pressure detecting device for detecting the line pressure, a torque detecting device for detecting the engine torque transmitted to the drive pulley, a torque allowance computing device for computing a maximum allowable belt transmitting torque which is effectively transmitted from the drive pulley to the driven pulley through the belt under the line pressure detected by the line pressure detecting device, and the torque control device for controlling a torque transmitted from the drive pulley to the driven pulley to a value smaller than the maximum torque when the engine torque detected by the torque detecting device is greater than the maximum torque. When a line pressure of a hydraulic control circuit therefor is unduly lowered, the slip between the drive and driven pulleys can be effectively suppressed.

What is claimed is: 
     
       1. A control system for a steplessly variable transmission comprising; a drive pulley and driven pulley formed with V-shaped grooves respectively,   an endless belt engaged with the both grooves of the pulleys and tensioned between the pulleys to transmit an engine output torque therethrough,   cylinder means provided on the pulleys for being subjected to a hydraulic pressure to vary an effective diameter of the pulleys so that a speed ratio of the transmission is continuously changed,   hydraulic pressure supply means for introducing the hydraulic pressure into the cylinder means,   line pressure control means for controlling a line pressure of a hydraulic circuit which controls operation of the cylinder means,   line pressure detecting means for detecting the line pressure,   torque detecting means for detecting the engine torque transmitted to the drive pulley,   torque allowance computing means for computing a maximum allowable belt tramsitting torque which is effectively transmitted from the drive pulley to the driven pulley through the belt under the line pressure detected by the line pressure detecting means,   comparing means for comparing the maximum allowable belt transmitting torque with the torque detected by the torque detecting means, and   torque control means for controlling a torque transmitted from the drive pulley to the driven pulley to a value smaller than the maximum torque when the engine torque detected by the torque detecting means is greater than the maximum allowable belt transmitting torque.   
     
     
       2. A control system as recited in claim 1 wherein the torque control means reduces an actual torque transmitted from the drive pulley to the driven pulley. 
     
     
       3. A control system as recited in claim 2 wherein the torque control means controls the hydraulic pressure to increase the speed ratio. 
     
     
       4. A control system as recited in claim 1 wherein the torque control means reduces the engine output. 
     
     
       5. A control system as recited in claim 4 wherein the torque control means retards an ignition timing. 
     
     
       6. A control system as recited in claim 1 wherein the engine output torque is obtained based on a throttle valve opening and an engine speed. 
     
     
       7. A control system as recited in claim 1 wherein the speed ratio is obtained based on a rotation speed of the driven pulley and a throttle opening. 
     
     
       8. A control system as recited in claim 1 wherein the maximum allowable belt transmitting torque is obtained based on a rotation speed of the driven pulley and the line pressure. 
     
     
       9. A control system as recited in claim 8 wherein a minimum line pressure under a condition where the maximum allowable belt transmitting torque is equal to a drive pulley input torque which is transmitted to the drive pulley from the engine is provided based on the drive pulley input torque and the speed ratio.